This invention relates to a method for determining the optimal assignment (distribution) of locomotives on a railroad network, by taking into account various costs and penalty factors associated with the movement and distribution of the locomotives.
One problem faced by railroad operators is the assignment of a sufficient number of locomotives of different types to operate on a pre-planned schedule. For efficient and timely operation of a railroad, it is essential that the tractive power (i.e., locomotives) is distributed around the network as required to move people and freight. The required distribution is also dynamic with respect to time and constantly changing due to track outages, planned or emergency locomotive maintenance, weather conditions, etc.
The problem of locomotive assignment can be illustrated by the following simple example. A railroad moves grain from Nebraska to a port on the Gulf of Mexico. To move the grain south requires a certain amount of tractive effort provided by the railroad locomotives. However, moving the empty cars north back to Nebraska, requires far less locomotive power because the cars are empty. The result is an accumulation of tractive power at the Gulf of Mexico port. To minimize its costs, the railroad desires to move the locomotives back north to Nebraska in a cost efficient manner. For example, the locomotives could be put into service moving freight on a different route such that they eventually find their way back to Nebraska, where they can again be put into service hauling grain south.
Each railroad has at least one locomotive dispatcher with the responsibility of ensuring that each terminal has the correct number of locomotives to move the freight on schedule. The locomotives must be in the yard and ready to depart on time. To carry out this function, the dispatcher must assess the location, movement, and availability of each locomotive and then predict when and where it will be available for the next train consist. For example, the dispatcher may be operating 16 hours ahead in planning the movement and availability of locomotives.
The cost of moving a locomotive either with a train as active power, with a train in-tow, or by itself must be considered by the dispatcher. A penalty may be incurred by the railroad, under its contract with a customer if a train is not ready to depart on time because there is not sufficient tractive power to move it or if the train arrives late at its destination. The penalty applied to a late arriving train is actually a sum of penalties associated with each individual car. The penalty incurred for each car depends on the shipping commitment pertinent to that car. The penalty could be zero for cars that have arrived ahead of schedule and could be very large for cars that are significantly behind schedule. In some situations, it may actually be cheaper for the railroad to pay the late penalty than to incur the cost of moving a locomotive to meet the schedule.
The dispatcher must also give consideration to various circumstances that may interrupt the smooth flow of freight on the railroad network. The dispatcher cannot exceed the locomotive capacity of each yard, and the traffic on each railroad corridor must be established to ensure that at any given time there are not an excess number of trains travelling the corridor, which would slow down the operation of all trains on the corridor. Locomotives must undergo various levels of inspections at predetermined intervals. Simple inspections are performed in the yard, while more thorough inspections must be performed in an inspection shop. The timing and duration of these inspections must be accounted for in determining the optimum distribution of locomotives in the network. Further, the productivity of each inspection shop will determine the length of time the locomotive is out of service.
Locomotives are segregated into power classes, with each power class having a different power/speed relationship, that is for a given speed the locomotive can generate a given amount of tractive power. The tractive force capacity influences the train consists to which the locomotive can be assigned. Obviously, heavier trains require the use of locomotives capable of pulling heavier loads. Assignment of locomotives must also be accomplished with consideration to track rail and load bearing capacities. Locomotives that are too heavy can damage the rail or the roadbed.
Railroad dispatchers communicate with the locomotive engineers by sending signals through the rails and through over-the-air communications links. There are two popular signaling systems referred to as Centralized Train Control (CTC) and Automated Block Signaling (ABS). For both the CTC and ABS signaling systems the track is divided into sections or segments, with each segment having a detection circuit. When a train enters a section, it closes the electrical circuit and sensors identify that a train occupies the section. Centralized Train Control comprises a system of computers that read the track sensor status and transmit data regarding train positions to a central dispatcher. The central dispatcher (either a computer or a human operator using a computer) schedules and controls train movements by setting the color of each individual traffic signal based on train position and specific railroad operational rules. In a centralized train control system, all signals in an area are controlled from one central point. In the ABS system, traffic signals colors are controlled by the train on the track. The color of the traffic signal at the beginning of the occupied segment is red, the color of the traffic signal on the previous segment is yellow, and the color of the signal on the next previous segment is green. Thus, the color of the signal is determined from the status of a single track segment.
In places along the railroad track where there is reduced visibility (narrow curves, steep slopes, etc.) secondary signals are installed. The secondary signals xe2x80x9crepeatxe2x80x9d or xe2x80x9cpredictxe2x80x9d the color of the next primary signal using combinations of colors and light positions. In the ABS system, a secondary signal is always subservient to its main signal, while in the CTC system each signal is independently controlled and, therefore, can change its aspect. Both the ABS and CTC systems send signals identifying the status of the next primary signal through the rails to the locomotive. Special equipment installed in the locomotive cab receives the signals to advise the engineer of the color of the next traffic signal before it becomes visible. Unfortunately, the systems use a different code of electrical signals for transmitting the signaling information.
Some locomotives are equipped to respond only to CTC signaling, others only to ABS signaling, and some locomotives to both. Occasionally, a locomotive may have no on-board signaling capability. Because the signaling system advises the engineer as to the conditions of the rail, occupancy of subsequent rail blocks along the path of travel, etc., the lead locomotive must have a signaling system capable of interfacing with the signaling system of the track over which it travels.
As can be seen, the scheduling and dispatching of locomotives is an extremely complex problem that can cause significant inefficiencies and extra costs to the railroad. In many situations, there simply is no solution to an under capacity problem other than running the locomotive, by itself, to the place where it is needed. This is an obvious cost to the railroad in terms of personnel, fuel, and wear and tear on the locomotive. Then there is the additional problem of finding a time slot in the corridor schedule so that the locomotive can travel to the terminal where it will later be needed.
In accordance with the present invention, the above described shortcomings of the locomotive dispatching process are obviated by a new and improved dispatching algorithm. The process utilizes a mathematical model of the locomotive assignment problem that assigns the locomotives in the network to various terminals at minimum cost and at the appropriate time. The present invention minimizes the costs of moving locomotives to achieve the desired distribution at the appropriate time and minimizes the penalties for delaying train departures. The present invention creates a plan for the distribution of all locomotives owned or controlled by a railroad with regard to power class and the cab signaling capabilities of each locomotive. The process can be executed using a sliding time window of any duration from the current moment to the future.
The present invention plans the movement of locomotives across the railroad network as either an active locomotive or as extra power connected to another train consist (i.e., in tow), and accomplishes optimal dispatching by considering the various parameters influencing the use and movement of locomotives across the network. These parameters include the power class and signaling capabilities of each locomotive. The process takes into account the number of locomotives needed at various points in the network and various costs associated with moving or holding the locomotives so that they are available when needed.